Digital printing machines can take on a variety of configurations. One common process is that of electrostatographic printing, which is carried out by exposing a light image of an original document to a uniformly charged photoreceptive member to discharge selected areas. A charged developing material is deposited to develop a visible image. The developing material is transferred to a medium sheet (paper) and heat fixed.
Another common process is that of direct to paper ink jet printing systems. In ink jet printing, tiny droplets of ink are sprayed onto the paper in a controlled manner to form the image. Other processes are well known to those skilled in the art. The primary output product for a typical digital printing system is a printed copy substrate such as a sheet of paper bearing printed information in a specified format. More development is underway of production printers that require inkjet direct marking onto cut sheet media. This includes UV gel inks, solid inks and aqueous inks.
The output sheet can be printed on one side only, known as simplex, or on both sides of the sheet, known as duplex printing. In order to duplex print, the sheet is fed through a marking engine to print on the first side, then the sheet is inverted and fed through the marking engine a second time to print on the reverse side. The apparatus that turns the sheet over is called an inverter.
FIG. 1 shows a state-of-the-art inkjet digital printing machine 20. Printer 20 includes a marking module or engine 22 having a plurality of ink jet print heads 23, disposed centrally on the marking engine 22, and facing downward. Printer 20 has a media path 24 along which the media sheet 34 moves. Printer 20 has a media path entrance 26 where sheets are fed into the printer by a media sheet feeder (not shown). Printer 20 also has a media path exit 28 where sheets leave the printer and are fed into a finisher (not shown). Printer 20 has an inverter 30 to turn the sheet over for duplex printing. A media sheet 34 leaving the inverter 30 follows arrow 32 back to the marking engine 22 for printing on the reverse side. Arrows 26 and 28 also indicate the process path direction, which is downstream from entrance 26 toward exit 28.
In cut sheet printing devices, under certain conditions, the lead-edge of the paper can curl up and have potential for separating from the marking transport and contact the print head. A sheet with out-of-spec flatness can occur when a duplexed sheet has a heavy ink image on the trail edge of side 1, which then becomes the lead edge when inverted and curls towards Side 2. This is most severe when the paper is thin, and the cross-process direction image is parallel to the grain direction of the paper (Example: letter size paper, grain-long, long-edge-feed).
In direct-to-paper ink jet marking engines, an ink jet print head is mounted such that the face (where the ink nozzles are located) is mounted a fixed distance from the surface of the media. The gap is typically 1 mm or less. Because the paper curl height can be several millimeters, it poses a risk to the print head because it can hit the print head face plate when it passes through the nominally thin gap that the print heads are spaced from the media.
Media sheets, typically paper, can curl or distort in several ways. LE curl is a concave upward bending along the process direction, such that the lead edge (LE) and the trail edge (TE) rise up off the transport, as shown in FIG. 2. The raised LE can impact multiple print heads across the paper width. Cross curl is a concave upward bending across the process direction, such that the left side and right side edges rise up off the transport, as shown in FIG. 3. The raised sides can impact multiple print heads. Both LE curl and cross curl are caused by ink on the first side of a duplex print that is inverted.
Dog ear is a crease with upward bending across the process direction at an angle across a corner, as shown in FIG. 4. The crease can impact multiple print heads downstream. This is caused by sheet damage in the paper path. Print head damage is severe due to greater pressure.
Cockle is multiple bumps or peaks distributed throughout the sheet. The bumps can impact multiple print heads downstream. Cockle is caused by the drying rate of ink, especially aqueous based inks.
The print head gap or distance of the print head to the sheet must be within 1 mm. The media sheet must pass freely under the print heads. The sheet must not contact the face of the print head, or serious damage will result. This requirement poses a challenge for cut sheet media since the corners, edges and body of the sheet may not be completely flat. The use of a hold down transport such as a vacuum conveyor helps to maintain the sheet flat and within the gap for the most part. Purposely delivering sheets with downward curl from the sheet supply tray also helps to hold the sheet flat. Nevertheless it is not guaranteed that a sheet is flat over the entire surface.
Ink jet print heads are very delicate and can easily be damaged if the face of the print head is contacted by the media which is passing nearby. The print heads are also very expensive and thus, it is very important to minimize any risk of damaging these print heads.
Accordingly, there is a need to provide a system, for use with inkjet printers, for detecting excessive media height due to sheet curl so that remedial action can be taken to prevent print head damage.
There is a further need to provide a media height detection system of the type described and that will match the high production rate of a digital printing machine.
There is a yet further need to provide a media height detection system of the type described and that is mechanically simple and robust, thereby minimizing cost.